Biochar: The gardener’s friend that could help slow global warming

A soil improvement practice common among household gardeners is offering another way to slow greenhouse gas (GHG) emissions that cause global warming.

It may also help improve farming and ease food shortages.

Biochar, the carbon-rich charcoal-like remnants of super-heated organic matter, is used by gardeners as a fertilizer to promote better growth of plants and flowers.

The hard material is added to soil to provide a resilient air trap that helps in the spread of oxygen and nutrients to plant roots. It also improves the drainage of water through soil, reducing flooding and stagnant pools that choke off vital nourishment to plants.

But biochar’s carbon sequestering properties are exciting scientists who think it can help reduce the escape of harmful GHGs such as carbon dioxide (CO2), methane and nitrous oxide from soil, and add to the armoury in the fight to bring down global temperatures.

Soil contains more than three times the amount of carbon found in the air, produced mostly from rotting leaves, trees and creatures, and is mostly lost to the atmosphere. However, if that biomass is burnt under oxygen-free conditions – a process called pyrolysis – the decay rate slows dramatically; some forms of biochar degrade over thousands of years.

The super-heated plant material also retains structures such as the xylem ‘veins’ in stems and leaves. These tiny vessels can host and promote the growth of microorganisms that act as carbon traps.

These processes help to lock carbon into the soil and dramatically slows the rate of GHG emissions.

Its usefulness has been recognized by decarbonization promoters.

Take the philanthropic office of financial giant Bloomberg, which is supporting seven cities worldwide, including Darmstadt in Germany and Minnesota in the US, to implement biochar programmes.

The cities, which also comprise Helsingborg in Sweden, Sandnes in Norway, Helsinki in Finland, as well as Cincinnati and Lincoln in the US, will each receive up to $400,000 to provide and promote biochar among citizens.

The programme hopes to sequester 10,000 tons (9,072 tonnes) of CO2 each year, equivalent to taking 6,250 cars off the road.

The usefulness of biochar depends on a number of factors. Different source materials offer variable rates of sequestration and the process of growing the feedstock and heating it to produce biochar also produces GHGs.

Nevertheless, governments and environmental organizations believe that biochar can be produced efficiently and cheaply at scale using waste that would ordinarily be composted or incinerated. Making it also produces heat energy that could be harnessed for community use.

What’s less understood is how well biochar boosts soil fertility. While aeration of the soil undoubtedly helps plants grow in denser substrates, the biochar doesn’t release any nutrients itself.

Scientists believe it can be a useful way to revive damaged soils but studies into this have not been running long enough to accurately assess this. Programmes like Bloomberg’s are helping to add knowledge.

If proven to be substantially beneficial, however, biochar use could be scaled up on farmers’ fields to help restore lost farmland and raise agricultural fertility and revive areas left barren by poor farming practices that have eroded soil.

According to the United Nations Convention to Combat Desertification (UNCCD), 20%-40% of global lands have been degraded by humans in this and other ways. The knock-on effect of that has been the destruction in those regions of biodiversity that is key to sustaining human life.

But biochar on its own won’t be enough to solve these problems. To help lower rising global temperatures, it needs to be used in conjunction with other natural carbon capture projects – such as reforestation – and technological developments like direct air capture of CO2.